A known steering apparatus includes mechanisms allowing an operation position of a steering wheel to be adjusted, which includes a telescopic mechanism allowing the operation position in a vehicle front-rear direction to be adjusted and a tilt mechanism allowing the operation position in a vehicle up-down direction to be adjusted. Specifically, related to the tilt mechanism, it is important that shakiness or looseness between a threaded shaft and a nut member is restricted, and a smooth slidability is ensured. For example, a slider mechanism constituted by a shaft (a threaded shaft) and a slider (a nut member) is disclosed in JP2000-280916A (hereinafter referred to as Patent reference 1). According to the slider mechanism suggested in Patent reference 1, the slider mechanism includes the shaft provided with a male threaded portion operably connected to an output shaft of a drive source and rotating therewith, and a female threaded portion formed in a substantially cylindrical shape. The female threaded portion is arranged around the male threaded portion so as to engage therewith, and moves with a target to be driven, while the female threaded portion is restricted from rotating in a rotational direction of the shaft. The slider mechanism disclosed in Patent reference 1 also includes a biasing member biasing the female threaded portion in a radial direction, and a pressing member which includes a contacting portion which is in contact with the male threaded portion and which receives a counter force of the biasing force of the biasing member so that the counter force acts as a contact force between the male threaded portion and the contacting portion. According to the slider mechanism suggested in Patent reference 1, the female threaded portion formed in the substantially cylindrical shape is biased in the radial direction by the biasing force of the biasing member and is securely in contact with the male threaded portion of the shaft. The counter force of the biasing force is received by the pressing member and the contacting portion of the pressing member is securely in contact with the male threaded portion of the shaft, and thus a backlash at an engaging portion of the threaded portions is forcibly removed. In addition, according to the slider mechanism suggested in Patent reference 1, deformation that occurs to a nut made of resin is not used as a measure to remove the backlash. Therefore, fluctuation in a sliding-friction torque at a screw mechanism portion, which is caused by changes in a Young's modulus of the nut and which is found in conventional techniques, does not occur. Further, it is suggested in the Patent reference 1 that, even in a case where resin material is used for the nut and metal material is used for the shaft, a tendency of tight fit at low temperatures is not likely to occur because the deformation of the nut is not utilized for removing the backlash unlike the conventional techniques.
A known electrically-powered steering column apparatus is disclosed in JP2009-248703A (hereinafter referred to as Patent reference 2). According to the electrically-powered steering column apparatus disclosed in Patent reference 2, a steering column rotatably supporting a steering shaft is driven so that a position of the steering column is adjustable in a tilt direction or in a telescopic direction. The electrically-powered steering column apparatus includes a motor, a rotating member driven by the motor to rotate and an axial-direction-displacing object for driving the steering column to move in the tilt direction or in the telescopic direction as the axial-direction-displacing object displaces itself, that is, moves, in the axial direction in response to rotation of the rotating member, wherein a coefficient of thermal expansion of the rotating member and a coefficient of thermal expansion of the axial-direction-displacing object are substantially equal to each other, a resin material is connected to a peripheral surface of one of the rotating member and the axial-direction-displacing object, and the resin material includes a threaded portion that is threadedly engaged with a threaded portion formed on a peripheral surface of the other one (which is made of metal) of the rotating member and the axial-direction-displacing object, which is made of metal. According to the apparatus disclosed in Patent reference 2, because the coefficient of thermal expansion of the rotating member and the coefficient of thermal expansion of the axial-direction-displacing object are substantially equal to each other, even in a case where an environmental temperature inside a vehicle cabin at which the electrically-powered steering column apparatus is arranged, amount of expansion or shrinkage is maintained substantially equal between the rotating member and the axial-direction-displacing object. Accordingly, for example, shakiness or looseness, and/or tight fitting are restricted from occurring.
According to the tilt mechanism disclosed in Patent reference 1, for example, the biasing member (spring) and the pressing member are needed, which increases the number of parts and assembly workload. Further, according to a first embodiment disclosed in Patent reference 1, a horizontal hole that is orthogonal to the shaft (threaded shaft) is provided at the resin-made female threaded member (nut member). Therefore, a degree of difficulty in resin-molding of the female threaded member increases, resulting in a cost increase. On the other hand, according to the electrically-powered steering column apparatus disclosed in Patent reference 2, dimensions of the threaded portions include variation within a certain range due to manufacturing variation of the rotating member (threaded shaft) and the axial-direction-displacing object (nut member). Because the initial variation is not absorbed, the looseness and tight-fitting that are attributed to the initial variation are not restricted. In addition, because one of the rotating member and the axial-direction-displacing object is made of resin and the other one is made of metal, a difference in coefficient of linear expansions arises between the rotating member and the axial-direction-displacing object that are threadedly engaged with each other (the difference in the coefficient of linear expansions does not decrease to zero), and thus it is concerned that the looseness and/or tight-fitting may be caused by temperature changes.
A need thus exists for a steering apparatus for a vehicle, which is not susceptible to the drawback mentioned above.